The Standard Model (SM) is a successful theory describing physics at least up to the electroweak scale, having survived more than 40 years of various experimental tests.
 However, there still remain a handful number of experimental and observational claims that indicate discrepancies from the SM predictions and consequently request extension of the SM: non-zero mass of neutrinos [1]}, anomalous magnetic moment of muon, \((g-2)_\mu \)  [2]}, [3]},
existence of dark matter (DM) [4]}, Primordial black holes may explain the whole DM. See, e.g., [5]}. and baryon vs. antibaryon asymmetry of the universe [6]}, [7]}.
There have been discussions of extending the
SM by gauging the lepton number,
e.g. \(L_\mu -L_\tau \)  or \(L_e-L_\tau \)  [8]}, intending to explain DM [9]}, [10]}, the muon anomalous magnetic moment \((g-2)_{\mu }\)  [11]}, [12]}, [13]}, [14]}, and more recently EDGES 21cm anomaly [15]}. The extension gives rise to a leptophilic light vector boson, dubbed as \(X\)  in this paper.
We note that the \(X\)  boson may couple to the quarks via interactions
with unknown heavy fermions that mix with SM quarks [16]}.
It may then be responsible for the recent anomaly from the KOTO experiment
in \(K_L \rightarrow \pi ^0 \nu \bar{\nu }\)  [17]}, [18]}, [19]}, [20]} and also
the anomaly from the Atomki experiment in \(^8{\rm Be}^*\) . The preferred mass of \(X\)  for these cases is in sub-GeV range; in particular \(m_X\simeq 17~{\rm MeV}\)  for Atomki [21]}, [22]}.
